Recursive Array functions in C

Question

The program finds the average length of words in a given input and prints the words greater than the average. Here's the program

#define STRING_LEN 80
#define ARRAY_LEN 3
void *emalloc(size_t s) {
    void *result = malloc(s);
    if (NULL == result) {
        fprintf(stderr, "Memory allocation failed!\n");
        exit(EXIT_FAILURE);
    }
    return result;
}


void numbers_greater(char **wordlist, int average, int n){

    if(n < ARRAY_LEN){
        int a = strlen(wordlist[n]);
        if(a>average){ 
            printf("%s", wordlist[n]);
        }
        numbers_greater(wordlist+1, average, n+1);
    }

 }


int main(void) {
    char word[STRING_LEN];
    char *wordlist[ARRAY_LEN];
    int num_words;
    double average;
    int i;

    while (num_words < ARRAY_LEN && 1 == scanf("%79s", word)) {
        wordlist[num_words] = emalloc((strlen(word) + 1) * sizeof wordlist[0][0]);
        strcpy(wordlist[num_words], word);
        num_words++;
    }


    average = 0.0;
    for (i = 0; i < num_words; i++) {
        average += strlen(wordlist[i]);
    }
    average = average / num_words;
    printf("%f\n", average);

    numbers_greater(wordlist, average, 0);



    for (i = 0; i < num_words; i++) {
        free(wordlist[i]);
    }
    return EXIT_SUCCESS;
}

The program works up until the "numbers_greater" method, giving a segmentation fault error. I'm new to C so I'm a little bit confused, the recursive method runs without an error without the strlen statement, but with the strlen statement (even if I set it to a static number like 2) it bombs out of the code. Am I traversing through the array incorrectly?

Sorry, it's supposed to be numbers_greater (I changed the names of the functions to improve readability) — John
– John, Commented Jul 31, 2017 at 2:29
A segfault often means that you are accessing an invalid pointer somewhere. I suggest that you use a debugger to step through your code to double-check what it is doing. — Code-Apprentice
– Code-Apprentice, Commented Jul 31, 2017 at 2:29
Please edit your question to show the current version of your code. Any inconsistencies will cause much confusion which will only frustrate anyone who wants to help you. It will also waste a lot of your time, not to mention those of us who are willing to help. — Code-Apprentice
– Code-Apprentice, Commented Jul 31, 2017 at 2:30

beeselmane · Accepted Answer · 2017-07-31 02:31:44Z

6

This line

numbers_greater(wordlist+1, average, n+1);

will increment both the wordlist pointer and the integer n. What you are doing, in effect is incrementing the value you are checking by 2 instead of by one, as you would like to do.

To eliminate the segfault, change this line to the following:

numbers_greater(wordlist, average, n + 1);

As a note, this could be done much easier using a simple for loop. Also, I did not read the rest of the code and there may be some other error that I missed, but this should eliminate the segmentation fault in this function.

answered Jul 31, 2017 at 2:31

beeselmane

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Comments

Theo Walton · Accepted Answer · 2017-07-31 02:38:45Z

2

Lets walk through

void numbers_greater(char **wordlist, int average, int n){

    if(n < ARRAY_LEN){
        int a = strlen(wordlist[n]);
        if(a>average){ 
            printf("%s", wordlist[n]);
        }
        numbers_greater(wordlist+1, average, n+1);
    }
}

I have an array of strings that can hold 3 things, and I put n = 1. It works the first time... but now we get to

numbers_greater(wordlist+1, average, n+1);

so now we have an array of strings that can hold 2 things because of the wordlist+1 . n = 2 now , so n < ARRAY_LEN is true, but wordlist[n] will result in reading the 3rd element inside an array that should only hold 2.

To fix this try

numbers_greater(wordlist, average, n+1);

answered Jul 31, 2017 at 2:38

Theo Walton

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